1. Field of the Invention
This invention relates to acid hardened photoresist compositions particularly suitable for i-line and g-line exposure and characterized by the presence of a buffer to neutralize strong photogenerated hydrohalide acids.
2. Description of the Prior Art
Photoresists are photosensitive films used to transfer images to a substrate. They are applied as liquid coatings to a substrate and dried to provide a film. After forming the film on the substrate, the film is exposed through a patterned photomask to a source of activating energy such as ultraviolet light to form a latent image. The photomask has areas opaque to activating radiation and other areas transparent to activating radiation. The pattern in the photomask of opaque and transparent areas defines a desired image that may be used to transfer the image to a substrate. A relief image is provided by development of the latent image pattern in the resist coating. The use of photoresist coatings is generally described, for example, by DeForest, Photoresist Materials and Processes, McGraw Hill Book Company, New York (1975), and by Moreau, Semiconductor Lithography, Principals, Practices and Materials, Plenum Press, New York (1988).
Known photoresist compositions can provide positive or negative imaged features having a resolution and size sufficient for many existing commercial applications. However, for many other applications, the need exists for new photoresists that can provide more highly resolved images of submicron dimension. It has been found in the art that to obtain submicron resolution, it is necessary to irradiate the photoresist with activating radiation at short wavelengths such as deep ultraviolet (DUV) or x-ray. Though photoresists are known that are responsive to short wave length exposures, most imaging equipment currently in use is capable of exposing the photoresist coating at the higher g-line and i-line wavelengths.
Photoresist compositions capable of imaging by DUV exposure include acid hardened photoresists such as those photoresist compositions disclosed by Feely in U.S. Pat. No. 5,034,304 incorporated herein by reference. An improvement to acid hardened photoresist compositions is disclosed in U.S. Pat. No. 5,128,232 to Thackeray et al. The DUV photoresists that are the subject of these patents typically comprise an alkali soluble phenolic resin, a photoacid generator that liberates acid upon exposure to activating radiation and an acid activated crosslinking or hardening agent whereby, upon exposure to DUV irradiation, the photoacid generator undergoes photolysis liberating acid that induces crosslinking and hardening of the photoresist coating in the exposed areas of the coating. The Feely patent discloses resin binders inclusive of phenolic resins. The Thackeray et al. patent is an improvement over Feely in that it discloses the use of resin binders that are copolymers of phenols and cyclic alcohols.
Acid hardening photoresist compositions such as those disclosed in the Feely and Thackeray et al. patents provide fine line, high resolution images. However, imaging equipment most prevalent in the industry is adapted for i-line or g-line exposure and the acid hardening resist compositions are insensitive at these exposure wavelengths. Consequently, efforts have been made in the industry to formulate acid hardening photoresist compositions that may be imaged by i-line and g-line exposure.
In U.S. Pat. Nos. 3,954,475 and 3,987,307, both incorporated herein by reference, there is disclosed a class of vinyl-halomethyl-s-triazine compounds capable of photolysis and halogen acid free radical generation upon excitation at a radiation within the wavelength of from about 330 to 700 nanometers, a range that includes both i-line and g-line wavelengths. It is disclosed in these patents that the described halogenated triazines may be used for formulation of light sensitive compositions. Photoresists utilizing the triazine photoacid generators have been formulated for i-line and g-line imaging. Such photoresists are disclosed in the literature, for example, in U.S. Pat. No. 4,189,323 and in European Patent Applns. 0 458 325; 0 519 298; and 0 621 509, each incorporated herein by reference. The photoresists disclosed in the patent and the European applications typically comprises an alkali soluble resin binder such as a phenolic resin, a crosslinking agent and the halomethyl triazine. In practice, it has been found that upon photolysis, the triazine liberates a strong halogen acid, but during use, the liberated acid migrates from exposed regions of a photoresist film into unexposed regions of the film. This causes crosslinking in both exposed and unexposed areas of the coating resulting in a decrease in differential solubility between exposed and unexposed regions of the coating. Decreased differential solubility makes development of the coating more difficult requiring the use of stronger developers. Use of a strong developer decreases image resolution.